1. Technical Field
This disclosure generally relates to physically unclonable functions, and more specifically relates to burn-in of delay chains for increased repeatability of physically unclonable functions.
2. Background Art
For security reasons, it is desirable to implement physically unclonable functions (PUF) in integrated circuits. A PUF is an electrical circuit function that is difficult to characterize and clone. A PUF is useful for challenge-response authentication type security in electronic systems. The PUF can be used in electronic systems to obtain different functions or security keys in two chips where the differences in the two chips are physically undetectable. Each PUF device has a unique and unpredictable way of mapping challenges to responses.
A silicon or physical PUF exploits random variations in a circuit produced during the manufacturing of the circuit device. The PUF circuit can use these random variations in the delay of circuit components to achieve an unpredictable mapping of challenges and responses. For example, given an input challenge, a race condition is set up in the circuit, and two transitions that propagate along different paths are compared to see which comes first. An arbiter, typically implemented as a latch, produces a logical “1” or a “0”, depending on which transition comes first. When a circuit with the same layout mask is fabricated on different chips, the logic function implemented by the circuit is different for each chip due to the random variations of delays. To achieve repeatability, the random delay needs to be greater than the sensitivity of the arbiter. If the random process variation does not cause a statistically reliable difference in the delay of the paths, then the PUF will have low repeatability. This means that variation in temperature, voltage, and other factors can change the response of the system and thus the challenge response of the circuit will not be useful due to low repeatability.